When unloading tractor trailer rigs and trucks at loading dock, truck operators are required by Department of Transportation & OCHOA regulations as well as other state and Federal regulations to chock the vehicle's wheels. When storing a trailer apart from a tractor, regulations also require a trailer's wheels be chocked.
Tractor operators often carry there own chocks in the cab for use under the required situations. In other circumstances, operators rely upon a loading dock having chocks available for use and don't carry their own.
Even for those operators that own chock(s), they often forget them when leaving a dock. In other circumstances, a chock properly chocking a first trailer at a loading dock may be pilfered by an operator of a second trailer after he/she has pulled into a neighboring bay on a dock.
Ultimately, safety suffers when a trailer's wheels are not properly chocked as the risk that the trailer will roll even if a couple of feet and cause injury to a person nearby is substantially increased.